late 14c., combinacyoun, from Old French combination (14c., Modern French combinaison), from Late Latin combinationem (nominative combinatio) "a joining two by two," noun of action from past participle stem of combinare (see combine (v.)).

1. A set containing a certain number of objects selected from another set. The number of combinations of r objects chosen from a set of n is n C r = n! / ((n-r)! r!) where "n C r" is normally with n and r as subscripts or as n above r in parentheses. See also permutation. 2. In the theory of combinators, a combination denotes an expression in which function application is the only operation. (1995-04-10)